lib EXERCISES IN THE PREPARATION OF TISSUES. 



acid, to preserve it. The tissue must be completely freed of 

 alcohol or chromium salts by irrigation, for 24 hours, in running 

 water ; it is then left in the gum for a like period, or longer. The 

 more thorough the impregnation the better will the mass cut when 

 frozen. It should then present a cheesy consistency. A microtome 

 is necessary, such as Williams', in which the freezing is accom- 

 plished by means of ether. The block of tissue, ^ to in. thick, 

 is placed on the brass plate with the adherent gum, is covered with 

 a non-conducting cap, and the ether spray is operated until the 

 whole mass is completely solidified. The razor is carried in a tripod 

 frame, the front foot being turned to regulate the thickness of the 

 section. Sections are then cut as thin as possible, and are trans- 

 ferred from the razor, upon which they accumulate, into a bowl of 

 water. Here they should remain until cleared of gum, which may 

 necessitate a change of water. The sections may be kept for later 

 use in 9QP- C - alcohol, provided well-stoppered bottles are used. 



Embedding in eelloidin. Tissue previously dehydrated in absolute 

 alcohol is placed in a thin solution of Scherer's eelloidin in equal 

 parts of absolute alcohol and ether, the preparation which at first 

 floats at the top sinks as the eelloidin solution penetrates. Let the fluid 

 thicken to a syrupy consistency and set it as follows. Construct a 

 receptacle of blotting paper, a lid less box held together with pins, or 

 wrap a piece of blotting paper round a cork so as to leave a projecting 

 tube ; wet the paper with water, fill with eelloidin, and put the 

 tissue in position, leave the whole exposed to the air until a film 

 forms on the surface, then float on 70 p<c - alcohol (or place in 

 chloroform) until firmly set. The mass when set should have a nearly 

 transparent and opalescent appearance, and can be preserved for future 

 use in 8QP- C - alcohol. 



When required for cutting, wash thoroughly in water (1 hour), and 

 then immerse in gum-freezing mixture for 10 to 20 minutes, and freeze 

 on a Williams' microtome. It is not desirable that the eelloidin itself 

 be frozen, it is enough if the gum fixes the mass to the plate of the 

 microtome, as the embedding material gives the necessary support. 

 The block held in the hand can be cut directly with a razor moistened 

 with alcohol. It must not be allowed to dry. The sections are placed 

 in water, stained, and mounted. To mount in balsam, clear with origanum 

 oil which does not dissolve eelloidin. 



